To a great extent, control over a moving sled is achieved by shifting one's weight. When turning, for example, one leans in the direction of the turn. In negotiating bumpy terrain, one constantly leans forward and backward in an effort to keep the sled on the snow.
In effect, the shifting of one's weight causes small, almost imperceptible perturbations in the position of an imaginary point, referred to as the center-of-mass. A rider's ability to quickly and precisely control these perturbations permits the performance of various stunts or acrobatic maneuvers on the sled.
A rider can shift the center of mass in three dimensions. By leaning to one side, the rider shifts the center-of-mass transversely. Leaning backwards or forwards shifts the center-of-mass longitudinally. Sitting up straight or crouching shifts the center-of-mass vertically.
Known sleds, such as that disclosed in Sellers, U.S. Pat. No. 4,666,171, feature a strap extending transversely across the rider's knees. This strap secures the rider's legs in a flexed position, and thereby limits the rider's ability to vertically adjust the center-of-mass.